In an aircraft, and more particularly in a helicopter, it is conventional to find actuators arranged in parallel or in series in flight control linkages. Such actuators arranged in parallel are usually referred to as trim actuators by the person skilled in the art.
Thus, a helicopter may include a trim rotary actuator associated with its longitudinal flight controls, a trim rotary actuator associated with its lateral flight controls, a trim rotary actuator associated with its collective pitch flight controls, and a trim rotary actuator associated with its yaw flight controls.
Each trim actuator then performs first and second functions. The first function is to improve pilot comfort by enabling the pilot to set a given control in a given position. For example, by setting the collective pitch trim actuator, the pilot no longer needs to maintain collective pitch by using the appropriate control lever and can therefore pay attention to other tasks. The second function of a trim actuator consists in enabling the neutral position of a flight control to be adjusted.
Furthermore, if the aircraft is fitted with an autopilot, the trim actuator can provide information to the autopilot system. A sensor measures the position of a mechanical element of a trim actuator and forwards this information to the autopilot system which deduces therefrom the position of the associated flight control. Under such conditions, a trim actuator is generally a rotary actuator having a motor that imparts rotary drive to an outlet shaft connected to the associated flight control by a link.
Furthermore, it should be observed that the trim rotary actuator includes a twistably-deformable structure suitable for determining a reaction force relationship for the flight control, where the reaction perceived by the pilot acting on the flight control is in fact delivered by the twistably-deformable structure.
Document FR 2 137 300 discloses a first type of trim rotary actuator. That rotary actuator has an outlet shaft secured to a link, i.e. a pivoted link forming part of a linkage. A lever is secured to a first end of the outlet shaft and a toothed sector is free to rotate about the second end of said outlet shaft. The toothed sector co-operates with a motor via stepdown gearing. In addition, the rotary actuator is provided with a twistably-deformable structure for generating a reaction force.
The twistably-deformable structure includes a helical torsion spring surrounding the outlet shaft with the terminal ends thereof being fastened to first and second gripper arms. These first and second gripper arms that are free to turn about the outlet shaft are provided with respective first and second plates.
Under such conditions, the lever and the toothed sector include drive means that cooperate with the twistably-deformable structure. More precisely, the drive means comprise first and second flat fingers that are secured respectively to the lever and to the toothed sector. Thus, the first and second flat fingers face each other so that each of them is clamped between the first and second gripper arms of the twistably-deformable structure.
When the pilot acts on a flight control, that causes the outlet shaft to move and consequently causes its lever to move. Since the toothed sector is stationary, the terminal end of the helical spring secured to the gripper arm in contact with the second flat finger connected to the toothed sector does not move, while, on the contrary, the terminal end of the helical spring secured to the gripper arm in contact with the first flat finger connected to the lever is caused to move by the lever. The helical spring is then stressed in torsion and generates a force that the pilot can feel.
When the pilot releases the flight control, the helical spring tends to return to its initial position corresponding to the neutral position of the flight control.
Document FR 2 330 591 describes a second type of rotary actuator. Its toothed sector is secured to a sheath that surrounds the twistably-deformable structure and the outlet shaft. The twistably-deformable structure then has first and second angular sectors interconnected by a helical spring and co-operating with drive means for the sheath and for the outlet shaft, i.e. abutments and pins.
Rotary actuators of both the first and the second types are effective. Nevertheless, it is necessary to ensure that there is no slack between the twistably-deformable structure and the actuator drive means. Any angular gap, even if very small, between the twistably-deformable structure and the actuator drive means can correspond to a significant movement of the aircraft flight control.
Thus, such angular slack is particularly awkward for a flight control provided with first and second rotary actuators arranged respectively in parallel and in series with the flight control linkage, the first actuator being a trim actuator and the second actuator conventionally tending to act on the flight controls at a frequency that is relatively high. If the first rotary actuator presents angular slack, then the pilot, will feel shaking in the flight control. Consequently, certain aircraft are provided with damper means that are heavy and expensive in order to attenuate the sensed shaking.
Similarly, since the autopilot measures the angular positions of elements secured to the helical spring of the twistably-deformable structure, the first and second gripper arms for an actuator of the first type or the angular sectors for an actuator of the second type, it is appropriate to minimize slack between the various elements in order to guarantee good accuracy.
A first solution consists in imposing extremely severe manufacturing tolerances. However that first solution turns out to be either insufficient or else too expensive. In accordance with document FR 2 438 585, a second solution consists in limiting angular slack after the rotary actuator has been manufactured, by including, a spacer. Nevertheless, that operation appears once more to be difficult.